2006 Highlander, 4Cyl, 110,000 miles, new plugs, new air filter. Code P0420 reset several times comes back. Read a lot from some knowledgeable people seems code is not defined as replace cat. I don't mind sending it to dealer for better tests than my scan tool, I just want to know what to ask for. I don't want to replace the O2 sensors since at that point costs add up.
Tires have proper air pressure, gas tank is full, and windows are up.
Thanks for any help